The present invention relates to systems and methods for the delivery of high power laser energy particularly for material processing operations such as heat treating, metal surfacing (e.g. alloying and cladding), welding, drilling, and cutting, as well as other machining operations.
Typically, laser beam delivery for material processing is accomplished by means of an ensemble of mirrors and prisms for beam steering. Mechanical constraints in a typical system impose practical limitations on the degree of beam steering available. Moreover, there are conflicting design requirements relating to the focal length of the focusing lenses employed: A relatively longer focal length lens allows the workpiece to be placed farther from the lens thus reducing the likelihood of damage to the lens system due to splattering. However, a longer focal length lens also increases the focused spot size, decreasing the available energy intensity.
Accordingly, fiber optic laser energy delivery systems have been developed whereby laser energy in the near infrared and visible spectrum is passed through a single optical fiber at power levels sufficient for material and metal processing. With an optical fiber which is basically light in weight, the laser beam may be moved in almost any direction at a rapid speed, for example by robotic control. In short, a fiber optic delivery system substantially increase the degrees of freedom of laser beam manipulation. Such systems are disclosed in commonly-assigned U.S patent application Ser. No. 450,951, filed Dec. 20, 1982, now abandoned, by M. G. Jones and G. Georgalas entitled "Laser Material Processing Through A Fiber Optic", and continuation application Ser. No. 714,660 filed Mar. 21, 1985, and in commonly-assigned application Ser. No. 608,042, filed May 7, 1984, by M. G. Jones, now U.S. Pat. No. 4,564,736 entitled "Industrial Hand Held Laser Tool and Laser System". The entire disclosures of application Ser. Nos. 450,951 and 608,042 are hereby expressly incorporated by reference.
As disclosed in those applications, beams from lasers such as 1.06 micrometer Nd:YAG and Nd:glass lasers, as well as 680 nanometer beams from ruby lasers and 630-730 nanometer beams from alexandrite lasers can be coupled to an optical fiber such that the laser energy enters the fiber, and is transmitted thereby. Using the systems described, average power levels of up to 200 watts can be transmitted through optical fibers. The shorter wavelengths of these particular lasers are preferable to the 10.6 micrometer wavelength of carbon dioxide lasers since laser spot size is proportional to laser wavelength. For equal power levels, a shorter wavelength laser in general provides a hotter focused beam, allowing the cutting of thicker material, at faster cutting rates.
There are a number of factors which must be taken in account when coupling a laser beam into an optical fiber. If the coupling is not highly efficient, then energy is released in the form of heat at the input end of the optical fiber, thereby destroying it.
In general, coupling requires that the laser beam be focused to a spot whose diameter is less than the optical fiber or, more particularly, the core thereof. (As employed herein, the term "optical fiber" is intended to mean that element of a fiber optic which actually carries the light beam, i.e., the core, and not the cladding and protective shielding.) At the same time, in order to ensure proper internal reflection during fiber optic transmission, the included angle of the focused beam must be less than twice the numerical aperture (NA) of the fiber. Numerical aperture is the sine of the half-angle over which an optical fiber can accept light rays, multiplied by the index of refraction of the medium containing the rays (which is 1.0 for air). For typical fibers, this included angle of the focused beam must be less than approximately 20.degree. to 24.degree..
There remain problems when attempting to transmit as much power as possible through a fiber having a diameter as small as possible. Prior to the present invention, the smallest fiber optic cable used when power levels exceed 100 watts was approximately 600 micrometers in diameter. Coupling 100 watts or more of 1.06 micrometer wavelength laser energy into a smaller diameter fiber becomes difficult because the focused spot size is too large. The flexibility of the 600 micrometer diameter fiber optic is limited to an approximately four-inch bend radius, which can be a limiting factor.
More particularly, in the design of systems for coupling laser energy into the end of a fiber optic, there are conflicting requirements. In particular, shorter lens focal lengths allow the focused spot size to be reduced; thus the spot can be focused on a smaller diameter optic fiber. However, at the same time, the shorter lens focal length leads to a greater cone angle. If the cone angle of laser energy entering the optical fiber becomes too great, then internal reflections within the optical fiber will not be total, and energy will escape through the walls of the optical fiber. For longer lens focal lengths, the spot size eventually becomes too large, and the maximum focal length is limited for this reason. Thus, for a given maximum entrance cone angle for a given fiber, the lens must have a focal length no less than a predetermined minimum.
Also, focused spot size is directly proportional to the product of the focal length of the focusing lens and the divergence of the laser beam. Thus, for a known fiber diameter and beam divergence, the lens focal length can be no greater than a predetermined maximum.
The divergence of typical rod lasers limits their ability to focus to small spot sizes sufficient to enter fiber with diameters less than 500 micrometers. As the power output level of the 1.06 micrometer wavelength laser increases (up to 400 watts), the beam divergence also increases. For example, 100 watts of laser power can be transmitted through a 800 micrometer fiber, but transmission problems occur when the power level is increased to 400 watts. The problems occur because of larger spot sizes resulting from the increasing divergence.